Filtration block for liquid metals and alloys, with a mechanical and physical-chemical effect

ABSTRACT

The invention concerns a filtration block for liquid alloys and metals, with a mechanical and physical-chemical effect. 
     Consolidating, for example by sintering, elements of an active mineral compound based on fluorides, results in a porous structure with a high degree of permeability, which acts both by a mechanical filtration effect and by a washing action in respect of the metal under the action of the active mineral compound which acts as a flux and provides for trapping inclusions. 
     Use for the final purification of aluminium, aluminium-base alloys, magnesium and magnesium-base alloys.

This is a division, of application Ser. No. 111,905, filed Jan. 14, 1980now abandoned.

The present invention concerns a filtration block with a mechanical andphysical-chemical effect, for liquid metals and in particular foraluminum and aluminum base alloys, magnesium and magnesium base alloys.

Both in the foundry casting of parts and the semi-continuous casting ofplates, billets or ingots and in continuous casting, it has beenrecognized that, in order to produce sound parts which are free fromdefects, it is necessary to treat the metal in order to remove therefromimpurities and in particular inclusions which would impair theproperties of the cast components. These operations may be purelymechanical in nature, such as filtration, or of a physical-chemicalnature such as bringing the metal into contact the fluxes.

In practice, the treatment operation comprises mechanical filtration anda washing operation by means of a flux which in most cases comprises oneor more molten alkali metal and alkaline-earth metal halides, whichfacilitates wetting the inclusions and combining them together in theform of dross.

The inclusions which are not wetted by the metal and which are wetted bythe fluxes, which arrive at a liquid metal-liquid flux interface whichis thus formed, are trapped at the interface and pass into the liquidflux. They are eradicated from the liquid metal in the course of thewashing operation.

A large number of patents disclose processes and apparatus which provideboth for washing the metal by a flux and filtration thereof throughporous materials.

The following may be mentioned in particular: U.S. Pat. Nos. 2,863,558,3,006,473, 3,010,712, 3,025,155, 3,039,864 3,172,757, 3,821,238,3,737,305 in the name of ALCOA; British Pat. No. 1,266,500 and FrenchPat. No. 2,061,246 to BACO (British Aluminium Company); French Pat. Nos.1,254,899 and 1,258,674 and British Pat. No. 1,148,344 to FOSECO; U.S.Pat. No. 3,227,547 to UNION CARBIDE and, more recently, U.S. Pat. Nos.3,893,917, 3,962,081, 3,947,363 and 4,075,303 to ALUSUISSE whichdescribe porous ceramic materials for filtering liquid aluminum.

These processes have been considered satisfactory for many years. At thepresent time however, the requirements made by the aeronauticalindustry, for thin strips for packaging, for thin sheets and for finewires put the producers of semi-manufactured articles of light alloysand metals under an obligation continuously to improve the qualitythereof, especially as the refinements made in respect of analyticalmethods have shown that microscopic inclusions which hitherto wereconsidered negligible could impair certain mechanical characteristicsand could increase the rejection rate and could sometimes render uselessand even harmful certain extreme heat treatments which are carried outin the vicinity of the melting point of the eutectic constituents of thealloys.

The present invention makes it possible to attain a degree ofcleanliness of the metal, which is markedly higher than that achievedwith the known processes.

The present invention concerns a filtration block for liquid alloys andmetals, with a mechanical and physical-chemical effect, having astructure with open pores, which is formed by the consolidation ofelements, of which at least the outside part is formed by an activemineral compound, the melting points of the elements and the activemineral compound being higher than the melting point of the metal to befiltered and in practice being higher than the temperature at which themetal to be filtered will be fed to the filtration block.

In the following specification, the expressions set out hereinbeforewill bear the definitions specified:

`metal to be filtered`: the metal or alloy which is intended to undergothe filtration treatment on the block according to the invention;

`active mineral compound`: a mineral salt, a mixture or a combination ofmineral salts, acting as a flux by acting by a chemical and/orphysical-chemical effect on the various impurities and inclusionspresent in the metals to be treated, its action finally resulting inremoval of the inclusions and impurities from the filtered metal;

`consolidation`: any process which makes it possible to form a porousstructure which is mechanically stable, at the filtration temperature,from the elements of active mineral compound; and

`elements`: the elements, the agglomeration of which forms the filterblock, irrespective of the form thereof: grains of regular geometricalshape, crystals, grains of irregular fracture, grains of elongate shapeand even needle-shaped grains, small rods or sticks, or fibres of acircular or any section.

The invention also concerns a process for the production of anelementary filtration block for liquid metals, with a mechanical andphysical-chemical effect, characterised by consolidating so as to forman open-pore structure, elements of which at least the outside partcomprises an active mineral compound.

The general idea of the present invention is to combine in a singlestructure a purely mechanical filtration action which makes it possibleto retain the inclusions and other foreign elements which are greater insize than the size of the passages in the filter, and aphysical-chemical purification action under the effect of an activemineral compound which acts as a flux which makes it possible to trapthe inclusions at the liquid metal-flux interface. The applicants havefound that this effect of physical-chemical purification by means of aflux, under some circumstances, was at least as effective with a solidflux in grain form as with a liquid flux, as was practised hitherto,such conditions being in particular the provision of a large contactarea between the solid flux grains and the liquid metal to be treated.The applicants also found that it was possible for the solid fluxelements to be consolidated, for example by sintering thus to produceporous materials which have both the shapes and the sizes of open poresand passages to permit mechanical filtration, in particular by trappinginclusions, by virtue of the distorted shape of the passages formedbetween said elements and by virtue of a large contact area with theliquid metal, permitting physical-chemical purification. It was foundthat the fluxes which are most suitable for the consolidation step andwhich moreover have a higher melting point than the temperature at whichthe metals and alloys to be purified are supplied were essentiallymineral fluorides and in particular alkali and alkaline-earth metalfluorides, and fluorides of magnesium and aluminum. Although the use ofsodium salts of theoretically possible, such use however is to berejected when treating aluminum and most of the aluminum-base alloys, asit gives rise to the danger of introducing sodium into the filteredmetal; it is known that this element has an unfavourable influence inparticular on hypersilicon-bearing Al-Mg and Al-Si alloys, even in verylow levels of concentration.

Of all the possible combinations as between the fluorides referred toabove, binary, ternary or even quaternary combinations whose meltingpoint will preferably be more than 800° C. may be mentioned.

This is the case for example with the following compositions which aregiven as percentages by weight, by way of non-limiting example:

    ______________________________________                                        CaF.sub.2 (45%)--MgF.sub.2 (55%)                                                                      melting point 980° C.                          AlF.sub.3 (8.5%)--KF (91.5%)                                                                          melting point 820° C.                          AlF.sub.3 (41%)--CaF.sub.2 (59%)                                                                      melting point 840° C.                          BaF.sub.2 (85%)--MgF.sub.2 (15%)                                                                      melting point 910° C.                          BaF.sub.2 (30%)--CaF.sub.2 970%)                                                                      melting point 1050° C.                         CaF.sub.2 (49%)--MgF.sub.2 (49%)                                                                      melting point 950° C.                          LiF (2%)                                                                      ______________________________________                                    

The nature and the form of the elements forming the filtration blockaccording to the invention may also vary within fairly wide limits.

Such elements may be homogeneous, that is to say, may be totally formedby the active mineral compound, or they may be heterogeneous, that is tosay, formed by an inert substrate covered with the active mineralcompound, or a porous inert substrate impregnated with active mineralcompound. The filtration block, taken as a unit, may comprise elementsof the same nature (homogeneous or heterogeneous) or a mixture ofelements of different natures.

The inert substrate may be selected from mineral compounds which do notreact with the metals to be treated. This is the case in particular withelectrofused alumina (corindon), electrofused alumina-zirconia compounds(corindons with zirconia) and different silico-aluminous products andsilicon carbide.

Carbonaceous substances such as graphite or certain cokes can also beused. Other compounds such as nitrides or oxynitrides of boron orsilicon could be used, but their high price makes them less attractive.

The shape and the size of the elements is also an important factor forcarrying the invention into effect. The shape of the elements determinesthe active surface area of the filtration block and the shape of thepassages. The active surface area which is minimal when using sphericalgrains is increased by using tabular or needle-shaped grains or grainsproduced by crushing balls or other simple geometrical shapes. Usingelements in the form of small rods or sticks or elongate fibres whereinthe ratio between the length and the mean transverse dimension may befrom about 5 to 200 for example makes it possible to produce filtrationblocks having a felt-like structure which is highly effective by virtueof the large active surface area, the particularly distorted shape ofthe passages and the high degree of porosity inherent in this type ofinterlacement structure.

Porous elements with a heterogeneous structure can be produced in knownmanner by agglomeration of a stable compound, for example alumina, witha hot-decomposable or sublimable compound which is removed in the hotconsolidation step or in the sintering step, and then by impregnationwith the molten active mineral compound.

Likewise, porous elements with a homogeneous structure may be producedby mixing the active mineral compound with a product which can beeliminated by heat, by volatilisation or by pyrolysis in theconsolidation step.

The size of the elements and their granulometric distributions is one ofthe factors which determines porosity and permeability of the filter,that is to say, in practice, its hourly filtration capacity.

It has been found for example that a filtration block according to theinvention which comprises elements in the form of grains measuring from4 to 8 mm, in a thickness of 40 mm, had a capacity for filtering liquidaluminum at about 750° C., which was of the order of 12 kg per square cmof surface area and per hour, with a pressure drop of from 10 to 20 mmof liquid aluminum. The porosity of such a filter is approximately 40%.

It is possible to use elements of much smaller sizes, down to a fewtenths of a micrometer, provided that the agglomeration process makes itpossible to achieve a sufficient degree of permeability to provide thehigh filtration flow rates required by industrial procedures.

Starting from elements in the form of small rods or sticks measuring 1mm in diameter and 15 mm in length, consolidation results in a felt-likestructure wherein the degree of porosity reaches from 80 to 90% and thecapacity for filtration of liquid aluminum at a temperature of 750° C.is about 25 kg per square centimeter of surface area and per hour, at athickness of 50 mm.

The filtration blocks according to the invention being formed by theconsolidation of elements, any shapes and dimensions required by thedifferent uses for which they are intended may be imparted thereto:plates of simple geometrical or relatively complex shapes, cylinders,tubes, crucible configurations, plugs or nozzles, which can bepositioned above a ladle, in a casting gate or at the inlet of a mould.In the latter case, it is possible to provide for the final treatment ofthe metal at the very last moment before it is shaped, at a position atwhich all causes of pollution and outside fouling have ceased to exist,thus to ensure that the cast or moulded components are of a qualitywhich has been impossible to achieve hitherto.

The process for production of the filtration blocks may be carried intoeffect in a number of different forms, according to the nature of theelementary grains.

A first embodiment of the process comprises melting the active mineralcompound, casting it in the form of a thin plate which is a fewmillimeters in thickness, crushing it and calibrating the elements bysieving to the desired granulometry.

The elements are then put into a mould and heated in a dry atmosphere toa temperature which is slightly lower than the melting temperature ofthe active mineral compound thereby to cause sintering thereof. Thesintering operation is effected without pressure. It is also possible toapply a moderate pressure during the sintering operation, with acorrelated reduction in the temperature and/or the duration of theoperation. Generally, consolidation by sintering is carried out in a dryatmosphere, avoiding any contact with a flame, at a temperature which isfrom 5° to 150° approximately lower than the melting point of themineral compound, for a period of time between 5 minutes and 1 to 2hours.

The resulting porous structure is highly stable and resistantmechanically.

An alternative form of the production process comprises compressing orpelleting the mixture of mineral compounds and then crushing thecompressed products or pellets in order to produce the elements whichare calibrated by sieving and agglomerated by sintering as describedhereinbefore.

Elements in the form of small rods or elongate fibres may be produced bydifferent known processes. By way of example, mention may be made ofgravity casting the molten active mineral compound through a graphitedie comprising a plurality of die orifices calibrated at the desireddiameter, followed by rapid cooling, or drawing through a die a pasteformed from the active mineral compound in the form of powder or in theform of fine crystals and a binding agent which can be removed in thehot condition by evaporation or by pyrolysis, in the consolidation step.In the latter case, the resulting product is fibres or rods with aporous structure.

When heterogeneous elements are used, a number of different forms can beenvisaged, depending on whether the elements are or are not porous. Thegeneral method comprises coating or impregnating the elements withactive mineral compound, for example by immersion in the compound in themolten state, and then, after cooling, effecting a fresh crushing stepfollowed by sieving for calibration purposes, shaping and sintering.

When the starting material is an active mineral compound in the form ofgrains of very small sizes, a few tenths of a micrometer to one or twomillimeters, in order to produce a structure which has a sufficientdegree of polarity it is possible to operate in accordance with knownprocesses to produce a preliminary mixture with a binding agent whichwill be removed in the consolidation step, for example a sublimableorganic compound such as camphor, naphthalene, or hexachloroethane or apyrolysable compound such as polyethylene or polyurethane.

Finally, another alternative form comprises firstly preparing by meansof any known process a porous or spongy structure with open pores, ofinert material, and impregnating it with an active mineral compound. Itis possible for example to consolidate by sintering grains of alumina soas to produce a porous structure, then impregnating the structure byimmersion in a bath of molten active mineral compound, and possiblyremoving the excess of mineral compound by vibration, suction,centrifuging, etc. It is also possible, in known manner, to start from aspongy element in the form of an organic substance which can be removedor destroyed by heat, for example expanded polyurethane foam, and toimpregnate it by immersion in a slop comprising the active mineralcompound and a solvent or a dispersant of aqueous or non-aqueous nature,and to carry out a baking operation which causes both destruction of thepolyurethane foam and consolidation of the active mineral compound. Thislast-mentioned process which is used for forming porous ceramicstructures was described in particular in U.S. Pat. Nos. 3,090,094 and3,097,930.

The following examples provide further details, in non-limiting manner,in respect of some methods of producing filtration blocks according tothe invention.

EXAMPLE 1

A binary mixture comprising 50% by weight of MgF₂ and 50% by weight ofCaF₂ was melted at a temperature of about 1000° C. The liquid was castin the form of a plate measuring 5 mm in thickness. After cooling, theplate was crushed and the crushed grains were calibrated, by sieving, atfrom 4 to 8 mm. 2% by weight of LiF was added to the calibrated grains,and intimately mixed by mixing over a prolonged period. The mixture wasput in a mould in the shape of a disc measuring 190 mm in diameter and40 mm in thickness, vibrated and compacted, and then sintered by beingheld at a temperature of 950° C. for 20 minutes.

After cooling, the resulting product was a disc having a degree ofporosity of about 40%, with a filtration capacity of 3.6 tonnes per hourof liquid aluminum at about 750° C., with a pressure drop of about 15 mmof liquid aluminum.

EXAMPLE 2

A ternary mixture comprising CaF₂ : 49% by weight, MgF₂ : 49% by weight,and LiF: 2% by weight, was melted at a temperature of about 1000° C.Operation was effected as in Example 1, the grains being calibrated atfrom 5 to 8 mm, the mould measuring 200×200×40 mm and the sinteringoperation being carried out by holding a temperature of 930° C. for 35minutes. This process resulted in a filtration block having a degree ofporosity of 40% and a filtration capacity of 5 tones per hour of liquidaluminum at about 750° C.

EXAMPLE 3

Alumina which is referred to as `tabular` alumina, with grains measuringfrom 5 to 6 mm, was impregnated for a period of 30 minutes in a bathcomprising a ternary mixture of 49% by weight of CaF₂, 49% by weight ofMgF₂ and 2% by weight of LiF at a temperature of 1050° C. The aluminawas then removed from the molten bath, drained, cooled, crushed andagain calibrated at from 5 to 6 mm.

The product was then shaped and sintered under the same conditions as inExample 2.

The resulting product was a filtration block with a degree of porosityof about 40% and a filtration capacity of 4.6 tonnes per hour of liquidaluminum at about 750° C.

EXAMPLE 4

Small rods or sticks with a section of about 1 mm and from 8 to 20 mm inlength were prepared by casting a molten mixture of 49% by weight ofMgF₂, 49% by weight of CaF₂ and 2% by weight of LiF through a graphitedie comprising holes which are 1 mm in diameter, followed by rapidcooling in a stream of dry air.

The small rods were disposed loose in a graphite mould in the form of asquare plate measuring 200×200×40 mm; vibration was applied in order tocause the rods to be interlaced, the rods then being agglomerated bysintering at a temperature of 920° C. for 30 minutes. The resultingproduct was a filtration block with a porosity of 85% and a filtrationcapacity of 10 tonnes per hour of liquid aluminum at about 750° C.

EXAMPLE 5

A filter block produced as in the preceding Example and in the form of athick disc measuring 60 mm in thickness and 40 mm in diameter was placedin the upper part of a feed tube of a low-pressure casting machine; theactive mineral compound was the ternary mixture comprising 85% by weightof BaF₂, 13% by weight of MgF₂ and 2% by weight of CaF₂, and the alloytreated was an alloy A-S 7G (an aluminum-base alloy containing about 7%of silicon and about 0.9% of magnesium); a very marked improvement wasfound with regard to the metallurgical soundness of the parts produced.

Using the filtration blocks according to the invention makes it possibleto produce aluminum, magnesium and alloys based on one or other of thesemetals, in a degree of purity which complies with the highestrequirements made by the users thereof (aeronautical industry, finewires, thin sheets for encasing and capacitors).

Use of the filtration blocks according to the invention also permitstreatment of other metals and alloys whose melting points are compatiblewith the melting point of the active mineral compound such as lead, tin,zinc, copper or alloys with such metals as their base. If necessary, thecomposition of the active mineral compound may be modified by addingother substances such as borates or carbonates.

The filtration blocks also have a very high degree of mechanicalstrength and a very high degree of cohesion. In continuous prolongedservice, no rupture or loss or breakdown in cohesion of elementarygrains or any particles whatever has ever been observed.

Before being put into service, the blocks must preferably be pre-heatedto a temperature close to that of the metal to be filtered, by any meansother than direct contact with a flame. In the event of a stoppage and,after cooling, the resumption of operation with the blocks is effectedin the same manner by simply reheating the blocks.

We claim:
 1. A process for producing a block for filtering select moltenmetals and alloys by mechanical and physical-chemical means,comprising:(a) placing within a mold a plurality of sinterable elementsof active mineral compounds having a select shape and size, andcomprising one mineral salt selected from the group consisting of CaF₂,MgF₂, AlF₃, LiF, KF and BaF₂, said mineral compounds having a meltingpoint exceeding that of the molten metal or alloys to be filtered, (b)heating within said mold said elements of active mineral compoundswithin a dry atmosphere to sinter and consolidate said elements into aporous block, and (c) cooling said sintered block to form said filterhaving pores selectively dimensioned to mechanically remove impuritiesfrom said molten metal and alloy while said mineral compound provides aphysical-chemical reaction with said molten metal and alloy for removalof additional impurities.
 2. The process of claim 1, comprising:(a)mixing said mineral components with a removable substrate, and (b)heating the mixture to a temperature which permits removal.
 3. Theprocess of claim 1 wherein said elements, prior to step (a) have beenformed by:(1) impregnating a spongy structure of pyrolizable organicmaterial with a mixture of said active mineral compound in a dispersantliquid, and (2) removing said dispersant liquid from said spongystructure.
 4. The process of claim 1 wherein said elements in step (a)have the form of a core of inert material and an outer coating of saidactive mineral compound thereon.
 5. The process of claim 1 whereinduring step (b):(1) compressing said elements to a pressure up to 150bars while heating said elements at a temperature of 5°-150° C. belowthe melting point of said active mineral compound for 5-60 minutes. 6.The process of claim 1 wherein said elements are in the form of smallrods or sticks having a length-width ratio of 5-200.
 7. The process ofclaim 1 where in step (b) said elements are heated to a temperature of5°-150° C. below the melting point of said active mineral compound for5-120 minutes.